A group-III nitride semiconductor light-emitting diode is fabricated by providing an electrode on a stacked layer structure having a pn-junction type light-emitting part comprising, for example, aluminum gallium indium nitride (AlXGaYIn1-X-YN, where 0≦X, Y≦1 and 0≦X+Y≦1). In the stacked layer structure, a buffer layer is generally provided for relaxing lattice mismatch between the substrate material and the group-III nitride semiconductor layer constituting the stacked layer structure, thereby growing a high-quality group-III nitride semiconductor layer (see, JP-A-2-229476 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)). In the stacked layer structure for use in a light-emitting device using a sapphire (α-Al2O3 single crystal) substrate, the buffer layer is exclusively composed of aluminum gallium nitride (compositional formula; AlαGaβN, where 0≦α,β≦1) (see, JP-A-2-229476 supra).
When a stacked layer structure uses an insulating material such as sapphire for the substrate, an electrode, namely, an ohmic electrode for supplying a device driving current to LED, comprising such a stacked layer structure is disposed on p-type and n-type conductive layers constituting the stacked layer structure (see, JP-A-6-260682).
FIG. 1 is a view schematically showing the cross-sectional structure of a conventional group-III nitride semiconductor LED 100 having a light-emitting part 10 of a pn-junction type double hetero (DH) structure comprising a sapphire substrate 101 having provided thereon, via an AlαGaβN (where 0≦α,β≦1 and α+β=1) buffer layer 102, a lower clad layer 103 composed of an n-type gallium nitride (GaN), a light-emitting layer 104 composed of gallium indium nitride (GaYIn1-YN, where 0≦Y≦1), and an upper clad layer 105 composed of p-type GaN. The substrate 101 is insulating and therefore, p-type and n-type ohmic electrodes 106 and 107 must be provided on the surfaces of the p-type conductive layer (p-type clad layer 105) and the n-type conductive layer (n-type clad layer 103), respectively, constituting the stacked layer structure. The electrode 107 is formed through a cumbersome processing of cutting a part of the light-emitting part 10, and since a part of the light-emitting part 10 is removed, the surface area of the light-emitting part 10 disadvantageously decreases. As a result, a group-III nitride semiconductor LED ensuring high light emission intensity cannot be provided.
In another conventional example, a stacked layer structure for use in a group-III nitride semiconductor blue LED is constructed using an electrically conducting crystal, such as gallium phosphide (GaP) or silicon, for the substrate (see, JP-A-2-275682). Also, a technique of forming a buffer layer from a boron phosphide (BP)-based material when constructing this stacked layer structure is disclosed (see, JP-A-2-275682 supra). In the case of a stacked layer structure using an electrically conducting crystal substrate, it is common to provide an electrode having a first conduction form corresponding to the conduction form of the substrate crystal on the back surface of the substrate and to dispose an electrode having a second conduction form opposed thereto on a stacked layer structure-constituting layer having conduction form opposite the substrate crystal (see, JP-A-10-247761). In this conventional example of an electrode configuration, the light-emitting part provided on the substrate surface does not need to be eliminated. Accordingly, the surface area of the light-emitting part is not reduced, which by itself is advantageous in obtaining a high intensity group-III nitride semiconductor LED.
FIG. 2 is a view schematically showing an example of the planarly structure of a conventional group-III nitride semiconductor LED 200 having a first conduction-type ohmic electrode (back-surface ohmic electrode) on the back surface of an electrically conducting substrate, and a second conduction-type ohmic electrode (surface ohmic electrode) on the surface of the stacked layer structure. The surface ohmic electrode 201 usually also serves as an electrode for wire-bonding (pad electrode) and is disposed only in the center part of one constituent layer 202 of the stacked layer structure (see, JP-A2-275682). In still another conventional example, a surface ohmic electrode is attached with a band-shaped electrode directly contacting the pad electrode in the center part (see, JP-A-11-168240).
The surface ohmic electrode has been heretofore mainly provided on an aluminum gallium nitride (AlXGaYN, where 0≦X,Y≦1 and X+Y=1) crystal layer (see, JP-A-6-314822). However, aluminum nitride (AlN) and gallium nitride (GaN) are greatly small in mobility compared with other group III–V compound semiconductors such as gallium arsenide (GaAs) and indium phosphide (InP) (see, Yasuharu Suematsu, Hikari Device (Photo-Device), 1st ed., 8th imp., pp. 28–29, Corona Sya (May 15, 1997)). For example, the hole mobility of aluminum nitride (AlN) at room temperature is 14 cm2/V.s, which is as low as about 1/30 in comparison, for example, with 500 cm2/V.s of an indirect transition-type boron phosphide (BP), (see, Photo-Device supra, pp. 28–29). Accordingly, by using a conventional electrode configuration means for providing an ohmic electrode only on a constituent layer, the device operating current cannot be satisfactorily diffused over a wide range of a light-emitting part, which is disadvantageous in obtaining a high-intensity group-III nitride semiconductor LED.
With respect to the current diffusivity, by taking into account the fact that the physical properties of the group-III nitride semiconductor are inferior to other group III–V compound semiconductors, a conventional technique discloses a means of using, a transparent metal thin film electrode as an ohmic electrode, directly connected to a pad electrode (see, JP-A-6-314822). For example, a transparent ohmic electrode composed of a gold (Au) thin film is provided almost over the entire surface of a gallium nitride (GaN)-related group-III nitride semiconductor layer which is doped with p-type impurity (see, JP-A-6-314822 supra). However, in this conventional technique, much of the light emitted from the light-emitting part is absorbed by the metal thin film constituting the electrode, and as a result, the light emission which can be displayed outside the LED is disadvantageously reduced in the intensity.
An indium tin composite oxide (ITO) film has a high transmittance to blue light, green light or longer wavelength light emitted from a group-III nitride semiconductor LED compared with the above-described metal thin film (see, Tomei Doden Maku no Gijutsu (Transparent Conductive Film Technology), 1st ed., 1st imp., pp. 97–101, Ohmu Sya (Mar. 30, 1999)). For more efficiently displaying the emitted light to the outside by utilizing this transmittance, a technique of constructing a group-III nitride semiconductor LED so that an electrically conducting transparent oxide crystal layer comprising ITO is provided as a contact layer (a window layer for transmitting the emitted light) of the gallium nitride (GaN) layer is known (see, (1) JP-A-49-122294, (2) JP-U-A-6-38265 (the term “JP-U-A” as used herein means an “unexamined published Japanese utility model application) and (3) Appl. Phys. Lett., Vol. 74, No. 26, pp. 3930–3932 (1999)).
In many conventional techniques, an electrically conducting transparent oxide crystal layer, such as ITO, is disposed particularly on a p-type GaN layer which is poor in current diffusivity due to difficulties in obtaining high hole concentration and high mobility (see, JP-U-A-6-38265 and Appl. Phys. Lett., supra). However, the ITO cannot exhibit good ohmic contact with the group-III nitride semiconductor crystal layer, and in the above-described case, the forward voltage (Vf) disadvantageously elevates (see, for more detail, Appl. Phys. Lett., supra, Vol. 74 (1999)).